RNA genes were once considered relics of a primordial “RNA world” that was largely replaced by more efficient proteins. More recently, however, it has become clear that non-coding RNA genes produce functional RNA molecules with important roles in regulation of gene expression, developmental timing, viral surveillance, and immunity. Not only the classic transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), but also small nuclear RNAs (snRNAs), small nucleolar RNAs (snoRNAs), small interfering RNAs (siRNAs), tiny non-coding RNAs (tncRNAs), repeat-associated small interfering RNAs (rasiRNAs) and microRNAs (miRNAs) are now believed to act in diverse cellular processes such as chromosome maintenance, gene imprinting, pre-mRNA splicing, guiding RNA modifications, transcriptional regulation, and the control of mRNA translation (Eddy, Nat. Rev. Genet., 2001, 2, 919-929; Kawasaki and Taira, Nature, 2003, 423, 838-842; Aravin, et al., Dev. Cell, 2003, 5, 337-350). RNA-mediated processes are now also believed to direct heterochromatin formation, genome rearrangements, and DNA elimination (Cerutti, Trends Genet., 2003, 19, 39-46; Couzin, Science, 2002, 298, 2296-2297).
The recently described phenomenon known as RNA interference (RNAi) is involves the processing of double stranded RNA into siRNAs by an RNase III-like dsRNA-specific enzyme known as Dicer (also known as helicase-moi) which are then incorporated into a ribonucleoprotein complex, the RNA-induced silencing complex (RISC). RISC is believed to use the siRNA molecules as a guide to identify complementary RNAs, and an endoribonuclease (to date unidentified) cleaves these target RNAs, resulting in their degradation (Cerutti, Trends Genet., 2003, 19, 39-46; Grishok et al., Cell, 2001, 106, 23-34). In addition to the siRNAs, a large class of small non-coding RNAs known as microRNAs (miRNAs, originally termed stRNA for “short temporal RNAs”) is believed to play a role in regulation of gene expression employing some of the same players involved in the RNAi pathway (ovina and Sharp, Nature, 2004, 430, 161-164).
Like siRNAs, miRNAs are believed to be processed endogenously by the Dicer enzyme, and are approximately the same length, and possess the characteristic 5′-phosphate and 3′-hydroxyl termini. The miRNAs are also incorporated into a ribonucleoprotein complex, the miRNP, which is similar, and may be identical to the RISC (Bartel and Bartel, Plant Physiol., 2003, 132, 709-717). More than 200 different miRNAs have been identified in plants and animals (Ambros et al., Curr. Biol., 2003, 13, 807-818).
In spite of their biochemical and mechanistic similarities, there are also some differences between siRNAs and miRNAs, based on unique aspects of their biogenesis. siRNAs are generated from the cleavage of long exogenous or possibly endogenous dsRNA molecules, such as very long hairpins or bimolecular duplexed dsRNA, and numerous siRNAs accumulate from both strands of dsRNA precursors. In contrast, mature miRNAs appear to originate from long endogenous primary miRNA transcripts (also known as pri-miRNAs, pri-mirs or pri-pre-miRNAs) that are often hundreds of nucleotides in length (Lee, et al., EMBO J., 2002, 21(17), 4663-4670).
The current model of miRNA processing involves primary miRNA transcripts being processed by a nuclear enzyme in the RNase III family known as Drosha, into approximately 70 nucleotide-long pre-miRNAs (also known as stem-loop structures, hairpins, pre-mirs or foldback miRNA precursors) which are subsequently processed by the Dicer RNase into mature miRNAs, approximately 21-25 nucleotides in length. It is believed that, in processing the pri-miRNA into the pre-miRNA, the Drosha enzyme cuts the pri-miRNA at the base of the mature miRNA, leaving a 2-nt 3′ overhang (Ambros et al., RNA, 2003, 9, 277-279; Bartel and Bartel, Plant Physiol., 2003, 132, 709-717; Shi, Trends Genet., 2003, 19, 9-12; Lee, et al., EMBO J., 2002, 21(17), 4663-4670; Lee, et al., Nature, 2003, 425, 415-419). The 3′ two-nucleotide overhang structure, a signature of RNaseIII cleavage, has been identified as a critical specificity determinant in targeting and maintaining small RNAs in the RNA interference pathway (Murchison, et al., Curr. Opin. Cell Biol., 2004, 16, 223-9). Both the primary RNA transcripts (pri-miRNAs) and foldback miRNA precursors (pre-miRNAs) are believed to be single-stranded RNA molecules with at least partial double-stranded character, often containing smaller, local internal hairpin structures. Primary miRNA transcripts may be processed such that one single-stranded mature miRNA molecule is generated from one arm of the hairpin-like structure of the pri-miRNA. Alternatively, a polycistronic pri-miRNA may contain multiple pre-miRNAs, each processed into a different, single-stranded mature miRNA.
Naturally occurring miRNAs are characterized by imperfect complementarity to their target sequences. Artificially modified miRNAs with sequences completely complementary to their target RNAs have been designed and found to function as double stranded siRNAs that inhibit gene expression by reducing RNA transcript levels. Synthetic hairpin RNAs that mimic siRNAs and miRNA precursor molecules were demonstrated to target genes for silencing by degradation and not translational repression (McManus et al., RNA, 2002, 8, 842-850).
Tiny non-coding RNA (tncRNA), one class of small non-coding RNAs (Ambros et al., Curr. Biol., 2003, 13, 807-818) produce transcripts similar in length (20-21 nucleotides) to miRNAs, and are also thought to be developmentally regulated but, unlike miRNAs, tncRNAs are reportedly not processed from short hairpin precursors and are not phylogenetically conserved. Although none of these tncRNAs are reported to originate from miRNA hairpin precursors, some are predicted to form potential foldback structures reminiscent of pre-miRNAs; these putative tncRNA precursor structures deviate significantly from those of pre-miRNAs in key characteristics, i.e., they exhibit excessive numbers of bulged nucleotides in the stem or have fewer than 16 base pairs involving the small RNA (Ambros et al., Curr. Biol., 2003, 13, 807-818).
Recently, another class of small non-coding RNAs, the repeat-associated small interfering RNAs (rasiRNAs) has been isolated from Drosophila melanogaster. The rasiRNAs are associated with repeated sequences, transposable elements, satellite and microsatellite DNA, and Suppressor of Stellate repeats, suggesting that small RNAs may participate in defining chromatin structure (Aravin, et al., Dev. Cell, 2003, 5, 337-350).
A total of 201 different expressed RNA sequences potentially encoding novel small non-messenger species (smRNAs) has been identified from mouse brain cDNA libraries. Based on sequence and structural motifs, several of these have been assigned to the snoRNA class of nucleolar localized molecules known to act as guide RNAs for rRNA modification, whereas others are predicted to direct modification within the U2, U4, or U6 small nuclear RNAs (snRNAs). Some of these newly identified smRNAs remained unclassified and have no identified RNA targets. It was suggested that some of these RNA species may have novel functions previously unknown for snoRNAs, namely the regulation of gene expression by binding to and/or modifying mRNAs or their precursors via their antisense elements (Huttenhofer et al., Embo J., 2001, 20, 2943-2953).
To date, the binding and regulatory sites within nucleic acid targets of the small non-coding RNAs are largely unknown, although a few putative motifs have been suggested to exist in the 3′UTR of certain genes (Lai and Posakony, Development, 1997, 124, 4847-4856; Lai, et al., Development, 2000, 127, 291-306; Lai, Nat. Genet. 2002, 30(4), 363-364).
One miRNA is also believed to act as a cell death regulator, implicating it in mechanisms of human disease such as cancer. Recently, the Drosophila mir-14 miRNA was identified as a suppressor of apoptotic cell death and is required for normal fat metabolism. (Xu et al., Curr. Biol., 2003, 13, 790-795).
Downregulation or deletion of other miRNAs has been associated with B-cell chronic lymphocytic leukemia (B-CLL) (Calin et al., Proc. Natl. Acad. Sci. USA, 2002, 99, 15524-15529), and human homologues of the murine mir-143 and mir-145 mature miRNAs were recently reported to be expressed and processed at reduced steady-state levels at the adenomatous and cancerous stages of colorectal neoplasia (Michael, et al., Mol. Cancer. Res., 2003, 1, 882-891).
Expression of the human mir-30 miRNA specifically blocked the translation in human cells of an mRNA containing artificial mir-30 target sites. In these studies, putative miRNAs were excised from transcripts encompassing artificial miRNA precursors and could inhibit the expression of mRNAs containing a complementary target site. These data indicate that novel miRNAs can be readily produced in vivo and can be designed to specifically inactivate the expression of selected target genes in human cells (Zeng et al., Mol. Cell, 2002, 9, 1327-1333).
Disclosed and claimed in PCT Publication WO 03/029459 are miRNAs from several species, or a precursor thereof, a nucleotide sequence which is the complement of said nucleotide sequence which has an identity of at least 80% to said sequence; and a nucleotide sequence which hybridizes under stringent conditions to said sequence. Also claimed is a pharmaceutical composition containing as an active agent at least one of said nucleic acid and optionally a pharmaceutically acceptable carrier, and a method of identifying microRNA molecules or precursor molecules thereof comprising ligating 5′- and 3′-adapter molecules to the ends of a size-fractionated RNA population, reverse transcribing said adapter containing RNA population and characterizing the reverse transcription products (Tuschl et al., Genes Dev., 1999, 13, 3191-3197).
Small non-coding RNA-mediated regulation of gene expression is an attractive approach to the treatment of diseases as well as infection by pathogens such as bacteria, viruses and prions and other disorders associated with RNA expression or processing.
Consequently, there remains a long-felt need for agents that regulate gene expression via the mechanisms mediated by small non-coding RNAs. Identification of modified miRNAs or miRNA mimics that can increase or decrease gene expression or activity is therefore desirable.
The present invention therefore provides oligomeric compounds and methods useful for modulating gene levels, expression, function or pathways, including those relying on mechanisms of action such as RNA interference and dsRNA enzymes, as well as antisense and non-antisense mechanisms. One having skill in the art, once armed with this disclosure will be able, without undue experimentation, to identify compounds, compositions and methods for these uses.